The present invention relates generally to the field of circuit breakers, and more particularly to a pivot joint for a movable contact arm in a molded case circuit breaker.
In general the function of a circuit breaker is to electrically engage and disengage a selected circuit from an electrical power supply. This function occurs by engaging and disengaging a pair of operating contacts for each phase of the circuit breaker. The circuit breaker provides protection against persistent overcurrent conditions and against the very high currents produced by short circuits. Typically, one of each pair of the operating contacts are supported by a pivoting contact arm while the other operating contact is substantially stationary. The contact arm is pivoted by an operating mechanism such that the movable contact supported by the contact arm can be engaged and disengaged from the stationary contact.
There are two modes by which the operating mechanism for the circuit breaker can disengage the operating contacts: the circuit breaker operating handle can be used to activate the operating mechanism; or a tripping mechanism, responsive to unacceptable levels of current carried by the circuit breaker, can be used to activate the operating mechanism. For many circuit breakers, the operating handle is coupled to the operating mechanism such that when the tripping mechanism activates the operating mechanism to separate the contacts, the operating handle moves to a fault or tripped position.
To engage the operating contacts of the circuit breaker, the circuit breaker operating handle is used to activate the operating mechanism such that the movable contact(s) engage the stationary contact(s). A motor coupled to the circuit breaker operating handle can also be used to engage or disengage the operating contacts. The motor can be remotely operated.
A typical industrial circuit breaker will have a continuous current rating ranging from as low as 15 amps to as high as several thousand amps. The tripping mechanism for the breaker usually consists of a thermal overload release and a magnetic short circuit release. The thermal overload release operates by means of a bi-metalic element, in which current flowing through the conducting path of a circuit breaker generates heat in the bi-metal element, which causes the bi-metal to deflect and trip the breaker. The heat generated in the bi-metal is a function of the amount of current flowing through the bi-metal as well as the period of time that that current is flowing. For a given range of current ratings, the bi-metal cross section and related elements are specifically selected for such current range resulting in a number of different current ranges for each circuit breaker. Some circuit breakers, industrial for example, provide for an indirectly heated bi-metal element. Electronic trip units are also used in some applications.
In the event of current levels above the normal operating level of the thermal overload release, it is desirable to trip the breaker without any intentional delay, as in the case of a short circuit in the protected circuit, therefore, an electromagnetic trip element is generally used. In a short circuit condition, the higher amount of current flowing through the circuit breaker activates a magnetic release which trips the breaker in a much faster time than occurs with the bi-metal heating. It is desirable to tune the magnetic trip elements so that the magnetic trip unit trips at lower short circuit currents at a lower continuous current rating and trips at a higher short circuit current at a higher continuous current rating. This matches the current tripping performance of the breaker with the typical equipment present downstream of the breaker on the load side of the circuit breaker. Again, electronic trip units can also be used. Because of the higher voltages and currents that must be interrupted, there is potential for damage to the components of a circuit breaker from the hot by-products of the electric arc interruption. During an electrical interruption, both gasses and small molten metallic particles are generated and expand outward from the electrical contacts into the arc chamber area of the circuit breaker and the contact arm heats up.
Another problem occurs in circuit breakers subject to high continuous current ratings. In a circuit breaker that is subject to high current, the overall size of the breaker must be larger in order to accommodate conductors with a larger cross section. This means that the crossbar must be longer. In addition, because greater pressure is required to maintain the contacts, the movable contact and the stationary contact, in a closed position a greater force is transmitted to the crossbar. Because of the longer length and the greater forces on the crossbar, the crossbar has a tendency to flex or bow along its length when the circuit breaker is xe2x80x9cONxe2x80x9d and the contacts are closed. In such situations, the crossbar flexes but the contact arm pivot remains stationary. As a result, the geometric relationship between the surfaces of the crossbar and the contact arm change which changes the amount of torque applied to the contact arm by the crossbar during normal operation or in an overload condition. Therefore, flexing of the crossbar can cause an unacceptable amount of variation in the pressure that must be applied to the contact arms and the dimensions of the contact arm assembly needed to maintain the proper mechanical and electrical coupling with the contacts.
Thus, there is a need for a molded case circuit breaker that will minimize or eliminate the effects of geometric and dimensional changes between the crossbar and the contact arm pivot. There is an additional need to overcome the physical misalignment of the contact arm to the stationary pivot caused by various factors, not the least of which is misalighed elements of the arm assembly due to manufacturing tolerances. There is also a need to prevent misalignment in the final contact arms of a circuit breaker closed. There is a further need for a molded case circuit breaker that can be easily reconfigured over a broad range of current ratings by utilizing interchangeable parts and additional parts with a minimum of unique parts.
The present invention provides a pivot joint for a movable contact arm assembly in a circuit breaker having an operating mechanism coupled to a line terminal, a trip mechanism and a load terminal with the load terminal coupled to a pivot mounting member. The pivot joint comprises a recess in a first sidewall of the contact arm with the recess co-axial with a mounting hole in the contact arm. A second recess is provided in a second sidewall of the contact arm with the second recess co-axial with the second mounting hole in the contact arm. The first and second recesses are co-axial with each other. A shaped washer, configured to fit in each of the first and second recesses, is provided and maintained in position by a mounting axle configured to engage each shaped washer, each mounting hole and the pivot mounting member. The contact arm is then free to pivot about the mounting axle. A spring, with the spring configured to engage both ends of the mounting axle, urges the contact arms against each shaped washer thereby completing the pivot joint. The two recesses formed in the contact arm can be in a facing relationship with each other and the shaped washers can be spherical.
The present invention also provides a molded case circuit breaker comprising a molded housing including a breaker cover, a first terminal and a second terminal mounted in the housing and a contact electrically coupled to the first terminal. An operating mechanism having a pivoting member movable between an xe2x80x9cONxe2x80x9d position, an xe2x80x9cOFFxe2x80x9d position and a xe2x80x9cTRIPPEDxe2x80x9d position is also mounted in the housing. An intermediate latching mechanism is mounted in the housing and coupled to the operating mechanism. A trip mechanism is selectively coupled to the operating mechanism and electrically connected to the second terminal. A moving contact arm and assembly including a pivot mounting member is coupled to the second terminal and the pivot member of the operating mechanism. A pivot joint for the moving contact arm comprises a recess in a first sidewall of the contact arm with the recess co-axial with the mounting hole in the contact arm. A second recess is provided in a second sidewall of the contact arm with the second recess co-axial with the second mounting hole in the contact arm, wherein the first and second recesses are co-axial with each other. A shaped washer, configured to fit in each of the first and second recesses are inserted in each recess. A mounting axle configured to engage each shaped washer, mounting hole and pivot mounting member is inserted in the mounting holes. The contact arm is then free to pivot about the mounting axle with a spring configured to engage both ends of the mounting axle and urging the contact arm against each shaped washer.
The present invention also provides a circuit breaker comprising a housing including a base, a means for connecting a load to the breaker mounted in the housing together with the means for connecting an electrical line to the breaker also mounted in the housing. The stationary contact is electrically coupled to the means for connecting the electrical line. A means for moving a contact arm coupled to a means for operating is mounted in the housing and having a pivoting member movable between an xe2x80x9cONxe2x80x9d position, an xe2x80x9cOFFxe2x80x9d position and a xe2x80x9cTRIPPEDxe2x80x9d position. The pivoting member is coupled to the means for moving a contact arm and with the means for operating coupled to the intermediate means for latching the means for operating. A means for tripping is coupled to the means for moving a contact arm and the means for connecting the load with the means for tripping in selective operative contact with the intermediate means for latching and the means for connecting a load is coupled to a means for mounting the contact arm. A means for pivoting the contact arm couples the contact arm with the means for mounting the contact arm. The pivot comprises a means for recessing in a first sidewall of the contact arm with the means for recessing co-axial with the mounting hole in the contact arm. A second means for recessing in a second sidewall of the contact arm is provided. The second means for recessing is co-axial with a second mounting hole in the contact arm wherein the first and second means for recessing are co-axial with each other. A means for reducing binding is configured to fit in each of the first and second means for recessing. A mounting axle configured to engage each means for reducing binding, each mounting hole and the means for mounting the contact arm is installed in the mounting holes wherein the contact arm is free to pivot about the mounting axle. A means for biasing, with the means for biasing configured to engage both ends of the mounting axle and urge the contact arm against the means for reducing binding, is installed.
The present invention also provides a method for improving the current carrying capacity of a pivot joint for a movable contact arm in a molded case circuit breaker. The circuit breaker has an operating mechanism coupled to a line terminal, a trip mechanism and a load terminal with the load terminal coupled to a pivot mounting member. The method comprises the steps of providing a recess in a first sidewall of the contact arm with the recess co-axial with the mounting hole in the contact arm. Providing a second recess in a second sidewall of the contact arm, with the second recess co-axial with the second mounting hole in the contact arm. The first and second recesses are co-axial with each other. Providing a pair of shaped washers and placing one of the shaped washers in each of the recesses. Providing a mounting axle and inserting the mounting axle into each mounting hole and engaging each shaped washer and the pivot mounting member. Then positioning a spring to engage both ends of the mounting axle and urging the contact arm against each shaped washer.